Abstract

1. An ATPase (adenosine triphosphatase) preparation obtained from pig brain microsomes by treatment with sodium iodide showed four apparently different ouabain-sensitive activities under various conditions. They were (a) ouabain-sensitive Mg2+-stimulated ATPase, (b) K+-stimulated ATPase, (c) (Na+,K+)-stimulated ATPase and (d) Na+-stimulated ATPase activities. 2. These activities showed the same substrate specificity, ATP being preferentially hydrolysed and CTP slightly. AMP was not hydrolysed. 3. These activities were inhibited by low concentration of ouabain. The concentration producing 50% inhibition was 0·1μm for ouabain-sensitive Mg2+-stimulated ATPase, 0·2μm for K+-stimulated ATPase, 0·1μm for (Na+,K+)-stimulated ATPase and 0·003μm for Na+-stimulated ATPase activity. 4. The ouabain-sensitive ATPase activities were inactivated by N-ethylmaleimide but the insensitive ATPase activity was not. 5. The three ouabain-sensitive ATPase activities were inhibited about 50% by 1mm-Ca2+, whereas the ouabain-sensitive Mg2+-stimulated ATPase activity was activated by the same concentration of Ca2+. The preparation was treated with ultrasonics at 20kcyc./sec. The 2min. ultrasonic treatment inactivated the ATPase activities by 50%. 7. The temperature coefficient Q10 was 6·6 for K+-stimulated ATPase activity, 3·7 for (Na+,K+)-stimulated ATPase and 2·6 for Na+-stimulated ATPase. 8. Organic solvents inactivated the ATPase activities, to which treatment the K+-stimulated ATPase was the most resistant. 9. The phosphorylation of the enzyme preparation became less dependent on Na+ with decreasing pH. This Na+-independent phosphorylation at low pH was sensitive to K+ and hydroxylamine as well as the Na+-dependent phosphorylation at neutral pH.